The idea is that they were always their identified gender even before they realized it or were comfortable saying it publicly. I get the sense from this post that you are tolerant, but not entirely informed. (Which is okay! When understanding stops being a process is when it stops being understanding.)

trans people are born their gender. There is a huge difference between cis people and trans people before transitioning with the same assigned gender at birth. Before they realize they are trans, they act as their gender, but convinced they are their assigned gender at birth so they try to do things they perceive as respecting that gender.

this is from my own experience and other trans people i have seen, so it probably doesnt apply to every trans person ever, but i dont think its that big of an assumption for a child to make

... Unless he transitioned before going to prom? Or maybe considers himself as always having been male before formally transitioning?

She was trying to be kind and inclusive. No need to grumpily look down on her because "uhm actually".

Which came first, the girl or the egg?

They were still themselves before they started transitioning, that part doesn’t change. They were always a boy if that’s what they’ve come to understand they are as part of transitioning.

Thank you. I actually had it backwards in my head until read your comment.